Escherichia coli is a gram-negative, rod-shaped bacterium. Although most strains of E. coli are benign and are found as normal intestinal flora of humans and other animals, some strains are pathogenic and can lead to disease. Different strains of pathogenic E. coli differ in their epidemiology, clinical course and potential for causing outbreaks of disease.
Pathogenicity has been linked to several serotypes, as defined by O and H antigens. Different pathogenic serotypes are associated with different clinical disease courses and have associated with them different levels of concern from the standpoint of public health. Several outbreaks of disease have been tracked to food and water borne sources of pathogenic E. coli. 
One serotype of E. coli in particular, serotype O157:H7, has been associated with several food and water borne outbreaks and is regulated as an adulterant in ground beef by the U.S. Department of Agriculture (USDA) with a zero tolerance standard. This serotype of E. coli is believed to have arisen from an O55:H7 parent strain, which then switched from O55 to O157 upon the transfer into the progenitor O55:H7 genome of the large virulence plasmid pO157, which contained the O157-rfb gene cluster as well as some additional genetic information (see, e.g., Wick et al., J. Bacteriol. 187:1783-91 (2005)).
Since E. coli is ubiquitous, and since serotype O157:H7 is highly pathogenic and tightly regulated, the ability to specifically detect and characterize E. coli serotype O157:H7 in a sample, even in the presence of other E. coli serotypes, is useful.
Published U.S. Patent Application No. 2011/0020823 and Sharma (Mol. Cell. Probes 20:298-306 (2006)) both teach the detection of E. coli O157:H7 through the simultaneous amplification of two sequence targets, neither of which is unique to E. coli O157:H7. However, the presence of both targets within a single strain is indicative of E. coli O157:H7. A problem arises when the sample being tested is not a pure culture, which is often the case in E. coli infections. In this case, it is possible for two different strains each to contribute one target for PCR amplification. Since both targets amplify, the response is erroneously interpreted as an E. coli O157:H7 response.
There is therefore a need for a method of detecting E. coli O157:H7 that does not potentially lead to false positive results.